The phone was so corrupted that ADB didn't see it. But the act of plugging it in while holding Volume Down triggered a failsafe. The screen flickered, and Fastboot appeared.

Keep holding the button until the screen changes to a white background with the Fastboot/Rescue Mode logo. 🛠️ Common Fixes if "Fastboot is Not Working"

Continue holding the button until the "Fastboot & Rescue Mode" screen (often featuring an Android mascot or text) appears. Verification

While holding Volume Down , plug the USB cable into your phone.

With the phone powered off and disconnected, Leo performed the magic combination:

fastboot devices